The present invention relates to an electric power steering system for obtaining a steering assist force for vehicle by means of an electric motor.
Heretofore, there has been proposed the electric power steering system arranged as follows (see, for example, Japanese Unexamined Patent Publication No. 2000-190855). Rotation of the electric motor is transmitted to a rotary element enclosing a rack shaft by means of a first bevel gear connected with an output shaft of the motor via a support shaft, and a second bevel gear meshed with the first bevel gear with their axes intersecting each other. Rotary motion of the rotary element is converted into linear motion of the rack shaft by means of a ball screw interposed between the rotary element and the rack shaft. The electric motor is driven into rotation in response to the rotation of a handle thereby supplying a steering assist force to a manual steering force for steering steerable road wheels coupled with the rack shaft.
Unfortunately, however, the conventional electric power steering system may sometimes encounter backlash at a meshing engagement area between the first and second bevel gears due to working precisions or assembly precisions of the first and second bevel gears. The backlash causes so-called rattling noises when a steering wheel is manipulated. The rattling noises may be conveyed to a passenger compartment to discomfort a vehicle operator and passenger.
Hence, there has been proposed an electric power steering system directed to the prevention of the backlash as follows. The electric motor is adapted for eccentric movement relative to a housing of the steering system. The eccentric movement of the electric motor provides adjustment of the position of the first bevel gear relative to the second bevel gear during the assembly of the system, thereby preventing the backlash (see, for example, Japanese Unexamined Patent Publication No.2000-190855).
However, it is quite difficult to avoid the backlash no matter how the adjustment is made during the assembly of the system as a countermeasure against the backlash, because tooth flanks of the bevel gear pair wear away from long-term use. Hence, there exists a need for cumbersome maintenance which includes periodical disassembly of the electric power steering system for backlash adjustment and the like.
As another countermeasure against the backlash, there is a common practice to interpose a shim between opposite surfaces of a support-shaft housing accommodating the support shaft and the first bevel gear and the electric motor in end-to-end relation such that the support shaft and first bevel gear may be axially moved in unison with the output shaft of the electric motor thereby adjusting the degree of meshing engagement between the first bevel gear and the second bevel gear.
In the above operation for backlash adjustment, however, the shimming must be usually repeated over and over again because a proper meshing engagement between these bevel gears cannot be achieved by a single shimming. Furthermore, each shimming involves disassembling the electric motor from the support-shaft housing and assembling the electric motor thereto. This makes the backlash adjustment complicated and cumbersome. In this approach, as well, the tooth flanks of these bevel gears wear away from long-term use of the system and hence, the occurrence of backlash is inevitable.
In addition, if the shimming involves a more than necessary amount of movement of the first bevel gear relative to the second bevel gear, a meshing engagement point between these bevel gears is shifted far away from a predetermined meshing engagement point. This results in the decrease of power transmission efficiency between these bevel gears.
There is another problem that the rotation of the support shaft entails vibrations, which propagate through a bearing to the support-shaft housing to produce noises.